1. Field of the Invention
The present invention relates to a recording apparatus such as a laser printer adapted to record an image by scanning a photoreceptor with a pulse modulated laser beam. More specifically, the present invention relates to a recording apparatus capable of correcting image distortion due to the difference in the incident angle and the moving speed of the laser beam between the central portion and the peripheral portion of the photoreceptor.
2. Description of the Prior Art
In a laser printer, a latent image of pixels is formed on a photoreceptor cylinder by a laser beam deflected by a polygon mirror rotating at high speed and, a visible image obtained by toner developing is transferred to a paper and then fixed thereon. The laser printer enables high speed and high quality (high resolution) printing and graphic recording, since high speed modulation of the laser beam is possible.
Generally, the laser printer has such structure as shown in FIG. 1, which is also employed in the embodiment of the present invention described in the following. The laser beam is emitted from a laser diode 1, deflected by a polygon mirror 3 and scans a photoreceptor 7. The incident angle of the laser beams changes depending on the position on the beam scanning line 8. In FIG. 4A, which is a schematic plan view illustrating the difference in the incident angles, while the incident angle .theta.b is a right angle at the point b in the central portion on the beam scanning line 8, the incident angles .theta.a and .theta.c are acute angles at the points a and c, respectively, near both ends of the beam scanning line 8. Consequently, the projected light images of the laser beam at the points a, b and c have their respective beam diameters Wa, Wb and Wc along the main scanning direction different from each other. More specifically, the beam diameters Wa and Wc near both ends are larger than the beam diameter Wb at the central portion. Therefore, assuming that the angular velocity of the laser beam is constant and the scanning is carried out for the same period of time with the laser beam turned on, then the images formed on the photoreceptor 7 will have beam widths of W4&gt;W6&gt;W5 as shown in FIG. 6, namely, the widths in the scanning direction of the images Pa, Pc and Pb at the points a, b and c are wider in this order. Therefore, an image distortion occurs, i.e. lines and dots become thicker and larger and the density thereof becomes higher in the end portions of the photoreceptor 7 compared with the central portion of the same.
The scanning speed of the laser beam on the beam scanning line 8 is faster at both ends than at the central portion, thereby increasing the above mentioned image distortion. However, the image distortion due to the difference in the scanning speed is generally corrected by employing a lens system including f.theta. lens 4 (hereinafter simply referred to as f.theta. lens 4). Meanwhile, a method for ensuring constant speed scanning electrically without using the f.theta. lens is proposed in Japanese Patent Laying-Open Gazette No. 33777/1985. In this method, the frequency of the switching clock of the laser beam is controlled so as to increase the speed of the clock near both ends of the plane being scanned and decrease the speed of the clock at the central portion. However, the difference in the incident angles can not be corrected in this method.